The present invention relates to the art of coating and sealing of metals and other solid substrate surfaces for preparing corrosion resistant non-chromium chemical conversion coatings for enhancing the corrosion resistance and adhesive bonding strength of the solid substrate surfaces, more particularly useful for aluminum, aluminum alloys, anodized aluminum, magnesium, zinc, titanium and titanium alloys, ferrous alloys, and galvanized steel, for example.
Aluminum and its alloys have been widely used in aerospace, automotive, and marine industry as structural materials because of their mechanical performance, and their low weight-to-strength ratio. The incorporation of alloying elements such as copper and the subsequent heat treatment lead to the formation of intermetallic compounds within the aluminum matrix. Unfortunately, the electrochemical potential difference between the intermetallic compounds and the aluminum matrix causes the aluminum alloys to be very susceptible to the localized corrosion such as pitting, especially in halide environments. In order to prevent the corrosion, therefore, aluminum surfaces are mostly treated with conversion coatings.
Passivation of aluminum and its alloys is typically done by the chemical or electrolytic conversion coating methods. Chemical conversion coatings are applied through chemical reactions, without externally applied electric potential, between the metal surface and bath solution which convert or modify the metal surface into a protective thin film with desirable functional properties. As a result of this reaction a corrosion-resistant outer layer on the base metal or its oxide becomes an integral part of the metal surface and often simultaneously produces a surface with enhanced paint adhesion. Therefore, the chemical conversion coating bath must include a suitable agent capable of reacting with both the aluminum and aluminum oxide surface film. Additionally, it must form a coating capable of forming an insoluble compound with aluminum and its alloying elements. The formed coating layers may provide corrosion and oxidation protection through galvanic effects or through simply providing a barrier layer to the surrounding corrosive environment. Unlike chemical conversion coatings, electrolytic conversion coatings, such as anodizing, require an electric current passing through the metal component immersed in a chemical bath. Anodic coatings formed on aluminum consist of a very thin non-porous barrier oxide layer, and a relatively thick porous layer. Without any post-treatment (or sealing), the corrosion performance of anodized aluminum largely depends on the properties of the very thin barrier oxide layer.
Among several chemical conversion coating types, hexavalent chromium, Cr(VI), or chromate-containing conversion coatings have been used for a long time because of their good adhesion and corrosion resistance. Similarly, hexavalent Cr(VI) based post-treatment solutions have been used to seal the anodic coatings in order to provide high corrosion performance and paint adhesion. However, solutions containing hexavalent Cr(VI) are highly toxic and adversely affect the environment and human health.
There is, therefore, a need for environmentally green conversion coatings and anodic coating sealers that can provide high corrosion resistance and increase the adhesive bonding strength. Although there are other conversion coatings and seals which do not contain chromium, their corrosion performance and paint adhesion characteristics are not as effective as the hexavalent Cr(VI) based conversion coatings and seals. Another drawback of chromated and non-chromated solutions is the formation of precipitates over time that leads to degradation of coating performance due to material loss in the bath.
Therefore, strict environmental regulations, tight waste disposal standards, and very low PEL (Permissible Exposure Limits) which are set and enforced by OSHA (Occupational Safety & Health Administration), have made the application of hexavalent Cr(VI) prohibitively expensive. Additionally, existing chromium-free corrosion inhibitors do not provide the corrosion resistance at desirable levels. Accordingly, there is a need for an environmentally friendly chemical conversion coating and anodic coating sealer that can provide good corrosion resistance and paint adhesion properties on aluminum substrates.